Resonant circuit



Patented Sept. 29, 1942 RESONANT CIRCUIT Dudley E. Foster, South Orange, N. J., assignor to Radio Corporation of America, a corporation of Delaware 7 Application May 7, 1941, Serial N0. 392,313

9 Claims.

The present invention relates generally to radio frequency signalling apparatus and more particularly to improvements in circuits employing tuned circuits having iron core coils.

It is an object of the present invention to provide an arrangement for obtaining a constant resonant impedance over a wide range of radio frequencies in circuits employing iron core coils.

Another object of the invention is to formulate the relationship between the constants and characteristics of iron cores and coils to the end that uniform amplification is obtained in vacuum tube circuits over a very wide range of frequencies.

Additional objects of the invention will appear in the following detailed specification when read in connection with the appended drawing.

In the drawing:

Figure 1 is a sectional view of a coil and core assembly which will be referred to hereinafter in explaining the invention;

Figure 2 is a schematic wiring diagram illustrating one way in which a coil and core assembly constructed in accordance with the present invention may be incorporated in a radio frequency circuit; and,

Figure 3 is a curve sheet which will be referred 60in explaining the invention. In Figure 3 there are shown curves of the loss factor per megacycle squared as a function of frequency for three different cores.

Referring more particularly to the drawing, Figure 1 illustrates a cylindrical coil 9 which is wound arounda sleeve M of insulating material, and a core l mounted within sleeve M. It is to be understood that the core [0 may be a ferro-magnetic core of the type usually employed in high frequency circuits. Suitable known cores for this purpose may be composed of comminuted iron or magnetite mixed with a binder and compressed or molded to the required shape.

In Figure 1, Z0 is the length of the coil, li is the length of the core, be is the radius of the coil and 111 is the radius of the core. Another dimension of the coil which is found useful in certain equations to be herein discussed is the length of the coil diagonal which is given by the expression J b. +l. where 120 and 10 represent the coil radius and coil length respectively, as above set forth.

From electrical engineering theory, it is known that the inductance of a coil without iron core is given by the expression where L =inductance of the coil without a core N=number of turns The inductance of such a coil with an iron core is given by the expression #:GfiBCtlVB permeability of core Li=inductance with'the iron core The power required to supply the coil losses is given by the expression W=I (Ro+Ri) where W=power required I=the current through coil Rt=resistance of coil without iron core R1=resistance added by the iron core The core loss may also be expressed by Wi Bi ViP where W1=power loss in the core Bi=fi11X density in the core Vi=v01ume of the core =specific loss factor for the iron core Now 41rNI r so that the resistance added by the iron core may be stated to be wL Q Where w=21r times operating frequency L=inductance R=radio frequency resistance of the coil In a coil employing iron where- Qi=Q of coil with iron m=ratio substituting the value of L0 above when dimensions are given in centimeters. Choosing the condition where the core substantially fills the coil, m becomes nearly equal to 1. If, in addition, we choose the dimensions of the coil so that which means that the diameter of the coil is approximately (more precisely 0.766 times) the length of the coil, and the core is equal to or greater in length than the coil, then which may also be written as Zi=wLiQi where Z1 is resonant impedance of the circuit with the iron core coil. Substituting in this equation the values of L1 and Q1.

u Z w -I-QQM B impedance of the circuit where Z0 is resonant without iron core.

Now if the effective permeability a of the core is larger than about 2, which is the case with all iron cores in use today as such cores have values of effective permeability ranging from 2 to upwards of 10 or 12, the first term of the denominator becomes much smaller than the second term and the tuned impedance is given closely by Now from the curves shown in Figure 3 it is seen that the specific loss factor per megacycle squared is substantially constant with frequency above 10 megacycles and for some irons above 6 megacycles. Therefore is a measure of the selectivity of the circuit since where Awis 21r times the bandwidth of the response characteristic at 0.707 of maximum response, so

where Awi=2 times bandwidth with iron Awo=21r times bandwidth without iron.

In order for the bandwidth with iron to be less than the bandwidth without iron, that is in order for the iron core coil to have better selectivity than the coil without iron must be less than Awo. In the usual case with ,1. large is nearly equal to ,lLp, in which case if up times 10 is less than Awo the selectivity is improved by use of iron. Now

n R0 air;

so the product of ,up time 10 must be less than the ratio of radio frequency resistance to inductance in order for the iron core to improve selectivity. The factor 10 is used when coil dimensions are in centimeters and resistances are in ohms and in that sense is merely a proportionality factor.

Having thus described my invention, what I claim is: a

1. In a radio frequency resonant circuit adapted to be tuned to frequencies above 6 megacycles, a coil, a magnetic core Within said coil and occupying substantially all of the space within the coil, said core having an effective permeability ,u and a loss coeificient B such that the product of a and 5 is less than the ratio of the radio frequency resistance to the inductance of the coil with the core removed whereby the circuit impedance is substantially constant over a range of frequencies above 6 megacycles.

2. The arrangement described in claim It wherein the length of the coil is at least 1.3 times its diameter.

3. The arrangement described in claim 1 wherein the core is composed of comminuted magnetic material mixed with a binder said core having an effective permeability of over 1.4.

4. The arrangement described in claim 1 wherein the length of the coil is approximately 1.3 times its diameter and the core is composed of comminuted iron, the core having an effective permeability which is at least 1.4.

5. In a resonant circuit which presents a substantially constant impedance over a predetermined wide range of radio frequencies above 10 megacycles, a cylindrical inductance coil having a length which is at least 1.3 times its diameter and an iron core substantially filling the space within said coil, said core having an effective permeability greater than 1.4 and a loss coeflicient which varies substantially as the square of the frequency.

6. In a high frequency resonant circuit having a resonant impedance which is substantially constant with respect to frequencies above a fre quency of approximately 8 megacycles, an inductance comprising a cylindrical coil whose length is at least 1.3 times its diameter and a core substantially filling the space within said coil, said core having an effective permeability which is at least 1.4 and a ratio of loss coefficient to the square of the frequency which is substantially constant.

7. In an iron core coil adapted to be used in a resonant circuit tuned to frequencies above 8 megacycles, in which the core occupies substantially all of the space within the coil, said core having an effective permeability [L and loss coefficient ,8 such that the product of ,u and p is less than the ratio of the radio frequency resistance to the inductance of the coil with the iron core removed.

8. In a resonant circuit, a cylindrical coil whose length is approximately 1.3 times its diameter, a magnetic core inserted fully within said coil and having a permeability greater than 1.5 and a substantially constant (i/w above about 10 megacycles, where l? is the specific loss factor of the core and w is 21r times the operating frequency, said core having a diameter slightly smaller than the inside diameter of said coil and a length at least equal to the length of the coil.

9. In a resonant circuit adapted to be tuned. to frequencies above 10 megacycles, an inductance coil having an iron core which occupies substantially all of the space within said coil, said coil having a length which is equal to or greater than 1.3 times its diameter, the impedance of said resonant circuit being substantially equal to is fl/ and being substantially constant for frequencies above 10 megacycles where L0 is the inductance of the coil without iron and e is the specific loss factor of the iron core.

DUDLEY E. FOSTER. 

